Addressing the House and Senate Judiciary Committee, they claim that the act's proposals:

"pose a serious risk to our industry's continued track record of innovation and job creation, as well as to our nation's cybersecurity."

SOPA is designed to wipe out so-called "rogue" websites that host copyright-infringing material by obliging service providers to monitor, block and delete sites judged to be linked to online piracy.

Google Chairman Eric Schmidt slammed SOPA and the Congress members who are sponsoring it, saying the solutions in the proposed bill are ‘draconian,” the Christian Science Monitor reported. “There's a bill that would require (Internet service providers) to remove URLs from the Web, which is also known as censorship last time I checked.''

Schmidt clarified that not supporting the bill does not mean he supports online piracy. ”We don't endorse it. Please don't do it. If you're doing it, stop. I hope that's very clear.''

It comes with a counterpart, the Protect IP bill, which, the BBC explains, aims to block sites linking to illegal content by having their domain names delisted from the internet's address books and removed from search engine indexes.

The act has been welcomed by Hollywood and other large copyright holders, but its opponents say it amounts to censorship.

According to the web companies' letter:

"The bills as drafted would expose law-abiding US internet and technology companies to new uncertain liabilities, private rights of action, and technology mandates that would require monitoring of websites."